1. Field of the Invention
The present invention generally relates to the art of electronic cameras, and more specifically to a charge-coupled-device (CCD) camera which is especially suited for low light level astrophotography.
2. Description of the Related Art
Photographs of astronomical scenes including the moon, planets, stars, galaxies and other celestial objects can be taken by a camera mounted at the primary focus of a telescope. An electronic camera using a CCD photosensor array as the photosensitive medium is highly preferred over a conventional camera which uses photographic film due to the CCD camera's much greater sensitivity, freedom from the tedious process of film processing and the ability to quickly display, modify and store electronic images using a computer. A photograph which requires hours of exposure time using photographic film can be taken by a CCD camera with an exposure time of several minutes.
If no compensation is made, an astronomical image which is focussed by the telescope onto the focal plane of the camera will move due to the rotation of the earth. For an exposure of more than approximately one second, the images of individual stars and other objects will appear elongated or streaked. For this reason, a motor or clock drive is commonly provided for moving the telescope in a path which is as closely opposite to the rotation of the earth as possible. This cancels the apparent motion of the image such that it appears stationary on the camera focal plane, and enables long exposures to be taken without streaking.
Although a clock drive can closely approximate the rotation of the earth, the mechanical tracking which it provides is never perfect. There are two general types of tracking errors. The first is periodic error in which the image on the camera focal plane appears to oscillate about a central point. This results from mechanical limitations in the gear mechanism of the clock drive. The second type of error is drift, and is caused by differences between the rotation rates of the earth and the clock drive and imperfect alignment between the earth and the axes of rotation of the telescope. The drift error results in streaking as described above, and accumulates over time. For example, doubling the exposure time results in twice as much drift and streaking.
Conventional clock drives are capable of producing images which are acceptably free of streaking for exposure times of approximately 30 to 60 seconds. For longer exposures, the astronomer must constantly monitor the view through the telescope and make manual drive corrections at intervals which are shorter than the maximum acceptable uncorrected drive time. This requires the astronomer not merely to be present during the entire exposure process, but to maintain concentration and diligence in constantly making small corrections.
This can become quite tedious for exposure times of an hour or more, and detract from the pleasure of astrophotography. It is clearly desirable to provide a feedback mechanism to automatically sense and correct tracking errors in the clock drive and enable unattended or "point and shoot" operation.
U.S. Pat. No. 4,958,224, entitled "FORCED CORRELATION/MIXED MODE TRACKING SYSTEM" issued Sep. 18, 1990 to R. Lepore et al discloses a method known as "video correlation tracking". A "live" image of a target which is being tracked and a "reference" image of the target are subjected to video correlation processing to determine the offset or registration error between the images caused by relative motion between the tracking optics and the target. A mechanical servomotor drive is adjusted by the amount of the computed offset to control the optics to track the target. The system disclosed by Lepore can also be operated in a "centroid" tracking mode, in which the offset between the live and reference images is determined by sensing and comparing the centroids of the two images.
A clock drive for astrophotography must move a telescope with much greater precision than is required for target tracking such as disclosed by Lepore. Gear backlash, inertia and other inaccuracies inherent in clock drives severely limit the precision which is attainable using video correlation or centroid processing to control a mechanical tracking drive. A system such as Lepore's if adapted for astrophotography would require an extremely large, accurate and expensive clock drive. This would place the system beyond the financial reach of most amateur astronomers, and would still produce only marginally clear images of faint objects in very deep space.